A wealth of knowledge has been amassed on the diverse roles played by oligosaccharides in biological systems. To further understand and exploit the activities of oligosaccharides, access is required to natural and nonnatural oligosaccharides, yet the synthesis of oligosaccharides remains one of the most challenging areas of chemistry. Glycosynthases represent a new class of mutant enzymes that are available to complement chemical synthesis and the use of glycosyl transferases as routes towards oligosaccharides. Glycosynthases are mutant glycosidases with a non-nucleophilic amino acid replacing the catalytic nuclecophile. They are hydrolytically inactive, but when presented with an activated donor sugar, that mimics the covalent glycosyl-enzyme intermediate, they efficiently condense donor sugar and acceptor sugar in high yield. This review illustrates the progress made since the discovery of the first glycosynthase in the author's laboratory (JACS, 1998, 120, 5583-5584). Several different glycosidases have been engineered to transfer a range of different donor sugars onto a wide variety of acceptor sugars, providing now synthetic routes to oligosaccharides. Advances have also been made in increasing the rates of transglycosylation of particular glycosynthases. This, in turn, has broadened the range of oliaosaccharides accessible via glycosynthase methodology.